Use a proper tty test.

lib/readline.js

@@ -40,7 +40,7 @@ function Interface (output, completer) {
 
   this.setPrompt("> ");
 
-  this.enabled = output.fd < 3;  // Looks like a TTY.
+  this.enabled = stdio.isatty(output.fd);
 
   if (parseInt(process.env['NODE_NO_READLINE'], 10)) {
     this.enabled = false;

test/disabled/test-readline.js

@@ -1,3 +1,5 @@
+// Can't test this when 'make test' doesn't assign a tty to the stdout.
+// Yet another use-case for require('tty').spawn ?
 common = require("../common");
 assert = common.assert;
 var readline = require("readline");